This invention relates to automated pattern inspection techniques and, more particularly, to the automatic inspection of microminiature masks and wafers that have multiply replicated patterns thereon.
Defects in the masks used in lithographic processes for fabricating microminiature components such as silicon integrated circuits can seriously affect the yield achieved in the manufacture of such components. As the size of these components decreases and as their complexity increases, the problem of inspecting the masks and the components themselves to assure adequate quality becomes increasingly difficult. Visual inspection by an operator of a mask or wafer having multiply replicated microminiature patterns is a time consuming, tedious and often inadequate procedure. As a result, considerable effort has been directed at developing automated inspection systems. In one such known system, patterns on adjacent chips of a mask structure are automatically compared by optically scanning corresponding parts of two adjacent patterns and subtracting the two resulting output signals to obtain an indication of any differences between the two compared parts. Such a two-chip-comparison system is described on pages 49- 52 of the Proceedings of the Microelectronics Seminar (Interface '73), Oct. 29- 30, 1973, Atlanta, Ga., sponsored by the Eastman Kodak Company.
In many instances of practical importance, gradual geometric changes may occur from pattern to pattern across the face of a mask or component wafer. Such changes arise, for example, from cumulative misalignments or from gradual thickness variations from pattern to pattern. Such gradual changes may accumulate to such a gross extent that the patterns at, say, opposite ends of a row of patterns will differ so significantly from each other as not to be acceptable. On the other hand, gradual acceptable changes may occur from pattern to pattern along a row of patterns. If a defect is defined as a unique occurrence in a pattern, an inspection scheme that identifies such minor gradual changes as defects would provide false error indications.
A simple two-chip-comparison inspection may fail to detect gross cumulative changes. Also, such an inspection procedure may fail to identify gradual acceptable changes and, as a result, falsely characterize such changes as defects.
Thus, the need has arisen for an automated inspection technique that provides a microscopic examination of individual patterns while at the same time providing an overall representation of multiple compared patterns so that changes that occur across the face of a mask or wafer may be properly characterized.